Every descent of the soul into this world has a specific Divine purpose. This is the case whether it is the soul’s first descent or a subsequent reincarnation. There are many aspects of gilgul haneshamot—reincarnation of the soul—that are complex and intricate beyond the scope of this response. However, to understand the general purpose for reincarnation, we can look to the three reasons outlined by Rabbi Yeshaya HaLevi Horowitz, as provided by the Kabbalists, in his classic work Shnei Luchot HaBrit (Shaloh).
1. There are some sins for which the cleansing in the spiritual realm alone does not suffice. Thus souls who have sinned and have not properly repented whilst alive, are sometimes forced to undergo a second round of life in this world as rehabilitation for sins previously committed. For the most part, rehabilitation occurs when the soul is faced with the same challenge to which it succumbed in its previous life, and overcomes it.
(This is also the reason why one should endeavor to do teshuvah (repentance) even if he knows that he has not done anything overtly wicked. After all, he may still have leftovers from a previous lifetime that need fixing. In fact, many have the custom to say, “I hereby forgive anyone who has angered or vexed me . . . in this incarnation or in another incarnation,” before their bedtime prayers every night.)
2. Reincarnation provides an opportunity for souls to perform those commandments that they were unable to do in a previous incarnation. Rabbi Isaac Luria (known as the Arizal) explains that the soul is comprised of 613 channels, which parallel the 248 limbs and 365 blood vessels of the human body.
These 613 channels attain eternal elevation when the soul fulfills all 613 commandments during its earthly descent. Usually a soul does not manage to fulfill all the commandments in one go, and must be repeatedly reincarnated until it has fulfilled them all.
3. There are some souls who do not descend for their own growth or perfection. Rather, the only reason they return to this earth is to benefit others. This can be to help out an individual or the entire generation, spiritually or materially.
Now that we know why souls generally reincarnate, it should be noted that in theory a soul can reincarnate as many times as needed until it fulfills its mission. Yet this can occur only if it rectifies at least a minute portion of its past while reincarnated. However, a soul that has reincarnated three times without having rectified anything at all is not reincarnated again.
This would imply that a soul that has reincarnated three times without rectifying its past has no other recourse for further rehabilitation and is forever doomed.
However, the Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, of righteous memory, explains that only reincarnations into human beings is limited to “three strikes and you are out.” These souls can still continue to be reincarnated, first as kosher animals, and then, in decreasing order, as non-kosher animals, plants, and even eventually as inanimate objects, as long as the need exists.
Therefore, even those souls who have transgressed sins so great that it is said that even languishing in Gehenna does not help rehabilitate them, have a way to be rehabilitated through reincarnation, and ultimately are reunited with their source.